Implement proper headless Web browser testing

[koute]

Right now if you launch cargo web test it will launch your tests in a new Chrome/Chromium instance it finds installed on your system. This is a huge hack which should be replaced by a proper headless browser (either headless Chromium or headless Firefox, or even both, because cross browser testing is important).

• [X] Write a driver for one of the headless browsers in Rust. (This shouldn't have any external dependencies like Node.js, etc.)
• [X] Replace the current cruddy way of running tests.
• [ ] Set up an automated way to build a package with our headless browser just as we're currently building Emscripten and Binaryen under Docker. (Linux only for now.)
• [ ] Make cargo-web automatically download and install it.

[whazor]

I would appreciate an integration with WebDriver (which allows ChromeDriver and GeckoDriver) as it would allow testing with multiple drivers and provides a common interface.